1. Field of the Invention
The present invention relates to magnetic heads for magnetic recording and magnetic recording apparatuses and, more specifically, to magnetic heads and magnetic recording apparatuses for microwave-assisted recording.
2. Description of the Related Art
There is a need for magnetic disk apparatuses, which are a type of magnetic recording apparatus, with higher recording densities. To ensure the write signal quality (S/N ratio) required for high-density recording, the size of magnetic grains forming magnetic recording media needs to be reduced with increasing areal recording density. Such magnetic grains, however, tend to suffer magnetization loss due to thermal fluctuations. To prevent this and maintain stable magnetization, the magnetic anisotropy energy of the magnetic grains needs to be increased. A material with high magnetic anisotropy energy forms a magnetic recording medium with high coercivity, which requires a strong write magnetic field for signal recording. This makes it difficult to perform recording because the strength of a magnetic field generated by a write head element is limited by the material and shape of the write head element.
To solve this technical problem, energy-assisted recording has been proposed, which applies supplementary energy to a magnetic recording medium during recording to decrease the effective coercivity, thereby assisting in signal recording on the medium (magnetization reversal of magnetic grains). A type of recording that uses a microwave magnetic field as a supplementary energy source is called microwave-assisted magnetic recording (MAMR) (see, for example, Jian-Gang Zhu, Xiaochun Zhu, and Yuhui Tang, “Microwave Assisted Magnetic Recording,” IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008).
There are known microwave-assisted magnetic recording systems, including systems (internal-oscillation systems) in which a microwave oscillator disposed near a write head element on a magnetic head generates a microwave magnetic field, and systems (external-oscillation systems) in which a microwave-generating element formed near a write head element generates a microwave magnetic field as it is supplied with a microwave excitation current by a microwave-signal generating circuit independent of the magnetic head. External-oscillation systems require microwave transmission lines for supplying a microwave excitation current to the microwave-generating element to be formed on a suspension and on a surface of a head slider on which the microwave-generating element is formed.
Japanese Unexamined Patent Application Publication No. 2010-73297 discloses a microwave transmission line provided on a suspension to reduce transmission loss of microwave signals. This technique controls the characteristic impedance of a microwave transmission line by modifying the positional relationship between shields disposed above and below the microwave transmission line and conductive posts connecting the shields to allow efficient transmission of microwave signals.
Japanese Unexamined Patent Application Publication No. 9-251813 discloses a technique for reducing transmission loss of write and read signals using a flexible printed cable (FPC) including signal lines, each composed of a plurality of conductors connected in parallel to reduce the inductance of the signal lines. This publication, however, does not mention the transmission or loss reduction of microwave signals.
These publications propose techniques for improving the characteristics of a transmission line formed on a suspension; they propose no technique for improving the microwave signal (excitation current) transmission characteristics of a microwave-generating element line formed on a side surface of a head slider (element-forming surface).
Effective microwave-assisted recording requires a certain microwave magnetic field strength. To efficiently generate a certain microwave magnetic field using a limited power supply to a magnetic recording apparatus, a microwave excitation current needs to be efficiently supplied to the microwave-generating element. This is also important for power conservation of the magnetic recording apparatus.
The above publications disclose only the control of the characteristic impedance of a transmission line on a suspension to reduce transmission loss for efficient transmission of microwave signals or write and read signals. However, only the control of the characteristic impedance of a microwave transmission line on a suspension is insufficient to efficiently supply a microwave excitation current to a microwave-generating element on a head slider, and the design of a line connected to a microwave-generating element formed on a head slider needs to be optimized.
The above publications, however, do not mention the optimization of a microwave element and a line formed on a head slider.